Apparatus for measuring properties of gases supplied to a patient

ABSTRACT

The gases temperature supplied to a patient when the patient is undergoing treatment such as oxygen therapy or positive pressure treatment for conditions such as Obstructive Sleep Apnea (OSA) or Chronic Obstructive Pulmonary Disease (COPD) is often measured for safety and to enable controlling of the humidity delivered to the patient. The invention disclosed is related to measurement of properties, particularly temperature (thermister), of gases flowing through a heated tube, supplying gases to a patient, which utilises the heating wire within the tube.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND Field

This invention relates to an apparatus for measuring properties, such astemperature and humidity, of gases being supplied to a patient.Humidifiers are commonly controlled by measuring the temperature of gasat two points, adjacent to the output of the humidifier and proximal tothe patient. This invention predominantly relates to the measurement oftemperature of gas supplied to a patient at a point proximal to thepatient.

Description of the Related Art

The gases temperature supplied to a patient when the patient isundergoing treatment such as oxygen therapy or positive pressuretreatment for conditions such as Obstructive Sleep Apnea (OSA) orChronic Obstructive Pulmonary Disease (COPD) is often measured forsafety and to enable controlling of the humidity delivered to thepatient. Measurement of temperature near the patient is commonlyperformed using a probe inserted into the breathing tube, such as thatof Fisher & Paykel Healthcare Limited, U.S. Pat. Nos. 6,272,933 and6,584,972. Such a temperature probe is connected to the humidifierthrough a cable that runs external to the breathing circuit Thisapproach has some drawbacks. In particular, the user must correctlyinstall the temperature probe. If the probe is not correctly installedthen the humidification system may malfunction which may increase riskto the patient. Existing end of breathing tube sensors require sensorwires to be run down the outside of the breathing tube. This lowersreliability of the sensors due to the vulnerability of these wires.Alternatively, if these wires are run down the inside of the breathingtube there would be an increase of the resistance to airflow and thehygiene of the breathing circuit would be lowered.

SUMMARY

It is an object of the present invention to provide a method ofmeasuring properties of gases supplied to a patient that goes some wayto overcoming the abovementioned disadvantages in the prior art or whichwill at least provide the industry with a useful choice.

Accordingly in a first aspect the present invention consists in anapparatus for measuring properties of gases being supplied to a patientcomprising:

a gases supply,

at least one delivery conduit including a heater wire for heating saidconduit,

wherein said heater wire is utilised in an electrical circuit todetermine said properties of said gases.

Preferably said electrical circuit is connected in series with saidheater wire and provides a measurement or enables a calculation of anindication of at least one of temperature, humidity, pressure andcomposition of said gases.

Preferably said electrical circuit is mounted and sealed on a printedcircuit board that at least partially extends into the gases supplied tosaid patient through said at least one delivery conduit.

Preferably said electrical circuit is at least partially moulded intothe wall of said delivery conduit.

Preferably said electrical circuit includes a sensing means with knownproperties at ambient temperature such that said sensing means can bematched with said at least one delivery conduit.

Preferably said sensing means is a temperature sensor.

Preferably said electrical circuit includes at least one measuring meansin series with said heater wire.

Preferably said at least measuring means is a temperature measuringmeans.

Preferably said temperature measuring means includes a thermistor anddiode in parallel and a reference resistor.

Preferably said thermistor and said diode are located at the end of saiddelivery conduit near to said patient and said reference resistor isincluded in said gases supply means.

Preferably said gases supply means includes a device to supply gas flow,such as a blower, and a humidifier to humidify said gases from saidblower.

Preferably said gases supply means is a humidifier.

Preferably said electrical circuit includes a gases property measuringmeans.

Preferably said gases property measuring means includes at least one ofa sensor, band pass filter or thermistor and at least one referenceresistor.

Preferably said at least one of a sensor, band pass filter or thermistorare located at the end of said delivery conduit near to said patient andsaid at least one reference resistor and at least one band pass filteris included in said gases supply means.

The invention consists in the foregoing and also envisages constructionsof which the following gives examples.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the present invention will now be described withreference to the accompanying drawings.

FIG. 1 is an illustration of a respiratory humidifier system that may beused with the method of the present invention of measuring temperatureof gases supplied to a patient.

FIG. 2 is a circuit diagram of the electronics enabling the measurementof the temperature of gases to a patient, where the circuit is utilisedwhen the system of the present invention is utilising DC heating andmeasuring voltages.

FIG. 3 is a circuit diagram of the electronics enabling the measurementof the temperature of gases to a patient, where the circuit is utilisedwhen the system of the present invention is utilising DC or AC voltagesfor the heating and signal voltages.

FIG. 4 is a cut away of a conduit including a circuit of the presentinvention on a printed circuit board and residing with the conduit inthe area of gases flow.

DETAILED DESCRIPTION

The present invention seeks to measure various properties, for exampletemperature or humidity, at the end of a gas delivery tube or conduitusing sensors mounted on a wire, such as a wire used for heating thegases flow through the tube or conduit, where the wire resides withinthe delivery tube or conduit. A heated tube with a heating wire such asthat described in Fisher & Paykel Healthcare Limited U.S. Pat. No.6,078,730 or any other similar tube and heating wire could be utilisedwith the present invention.

Referring to FIG. 1 a ventilation and humidifying system as might beused with the present invention is shown. A patient 13 is receivinghumidified and pressurised gases through a nasal cannula 12 connected toa humidified gases transportation pathway or inspiratory conduit 3 thatin turn is connected to a humidifier 8 (including humidification chamber5) supplied with gases from a blower 15 or other appropriate gasessupply means.

The inspiratory conduit 3 is connected to the outlet 4 of thehumidification chamber 5 that contains a volume of water 6. Thehumidification chamber 5 is preferably formed from a plastics materialand may have a highly heat conductive base (for example an aluminiumbase) that is in direct contact with a heater plate 7 of humidifier 8.The humidifier 8 is provided with control means or an electroniccontroller 9 that may comprise a microprocessor based controllerexecuting computer software commands stored in associated memory. Gasesflowing through the inspiratory conduit 3 are passed to the patient byway of the nasal cannula 12, but may also be passed to the patient byway of other patient interfaces such as a nasal or full face mask.

The controller 9 receives input from sources such as user input means ordial 10 through which a user of the device may, for example, set apredetermined required value (preset value) of humidity or temperatureof the gases supplied to patient 13. In response to the user sethumidity or temperature value input via dial 10 and other possibleinputs such as internal sensors that sense gases flow or temperature, orby parameters calculated in the controller, controller 9 determines when(or to what level) to energise heater plate 7 to heat the water 6 withinhumidification chamber 5. As the volume of water 6 within humidificationchamber 5 is heated, water vapour begins to fill the volume of thechamber above the surface of the water and is passed out of thehumidification chamber 5 outlet 4 with the flow of gases (for exampleair) provided from a gases supply means or blower 15 which enters thehumidification chamber 5 through inlet 16.

The blower 15 may be provided with a variable speed pump or fan 2 whichdraws air or other gases through the blower inlet 17. The speed of thevariable speed pump or fan 2 maybe controlled by a further control meansor electronic controller 18 which responds either to inputs fromcontroller 9 or to user-set predetermined required values (presetvalues) of pressure or fan speed, via dial 19. Alternatively, thefunction of this controller 18 can be combined with the other controller9.

A heating element or wire 11 is preferably provided within, around andthroughout the conduit or tubing 3 to help prevent condensation of thehumidified gases within the conduit. Such condensation is due to thetemperature of the walls of the conduit being close to the ambienttemperature, (being the temperature of the surrounding atmosphere) whichis usually lower than the temperature of the humidified gases within theconduit. The heater element effectively replaces the energy lost fromthe gases through conduction and convection during transit through theconduit. Thus the conduit heater element ensures the gases delivered areat an optimal temperature and humidity.

Such a heater wire is commonly driven either with direct current (DC) oralternating current (AC) and in both cases the heating voltage isusually switched on and off to control the power applied to the heatingelement. In the present invention the heating element 11, which is mostpreferably a wire, is used along with an electronic circuit to determineproperties of the gases supplied to the patient. The circuit (20 or 40in FIGS. 2 and 3) is preferably connected in series with the heater wire11. The circuit may be on a printed circuit board, or wired within ahousing that may be a plastic moulding in the gases flow, or a circuitboard that is at least partially moulded within the wall of the conduitor tubing 3. The properties that may be measured include temperature,pressure, gas composition and humidity. Two embodiments of the presentinvention are described below, one that operates using only a DC heatingvoltage and the other that can operate with a DC or AC heating voltage.

DC Heating Voltage

FIG. 2 shows a circuit 20 that may be utilised for carrying out themethod of measuring temperature of the present invention. When a DCheating voltage 25 is applied to the heater wire the diode 22 conductsand current flows through the heater wire 21, 28 and the heater wirefunctions as normal and provides heating to the delivery tube 3. Whenthe heating voltage 25 is switched off using switch 29, a measurementvoltage 26, which has opposite polarity to the heating voltage 25 isapplied to the heater wire. In this case, the current in the heater wire21, 28 does not flow through the diode 22 but flows through thethermistor 23 and through a reference resistor 24. The voltage acrossthe reference resistor 24 can then be measured at the output 27 and thetemperature of the gases determined. The voltage measurement 27 acrossthe reference resistor, 24, is converted to a temperature using a lookup table or an equation to calculate a value for temperature. This issimilar to a commonly used technique where the thermistor 23 forms apotential divider with the reference resistor 24.

More generally, the thermistor may be replaced by an impedance (forexample, a resistor and a capacitive sensor) for pressure or humiditymeasurement. Either the impedance can be measured by measuring thevoltage across the reference resistor 24 or the rise-time could bedetermined by looking at the voltage across the reference resistor 24 intime.

Part of the circuit 20 would be included in the delivery conduit 3 andin particular the diode 22 and thermistor 23 (in parallel with oneanother) are preferably placed in series with the heater wire 21, 28 ata point in the heater wire at or near the end 30 (nearest the user 13,see FIGS. 1, 2 and 4) of the delivery tube 3, for example they may beinterconnected on a printed circuit board, overmoulded with plastic forsealing and mounted in the gases stream through the delivery conduit asshown in FIG. 4. Furthermore, the circuit may be formed byinterconnected parts in a housing, for example, a plastic housing, thatprotrudes from the plastic wall of the delivery tube into the gases flowthrough the conduit, in order to measure that gases properties. Allother parts of the circuit 20 including the reference resistor 24 andthe switching circuitry 29 would be included in the control circuitry ofthe humidifier 8.

The thermistor's value can be chosen to have different resistance curveswith known properties at ambient temperature. The choice of a particularthermistor value for use with the circuit allows identification by thecontrol system of the present invention and matching of that thermistorvalue with a specific conduit or tubing 3. Such that differentthermistor values can be matched with a particular and appropriateconduit types and upon connection of the conduit to a humidifier orblower device, the control system can identify that thermistor and applythe appropriate control strategy to the heating of the conduit.

AC or DC Heating Voltage

The circuit shown in FIG. 2 is intended to be used when a DC heatingvoltage is used in conjunction with the heater wire, delivery conduitand system as shown in FIG. 1. An alternative embodiment of a circuit 40that would provide measurement of the gases properties, such astemperature and is suitable for AC and DC voltages, is shown in FIG. 3.A number of voltage signals 51, 52, 53, which are at differentfrequencies, are added together at an adder 50. These signals include atleast one heating signal 51 and at least one measuring signal 53. Thecombination of these signals passes down the heater wire 44, creatingcurrents (heating and measuring) in the heater wire 44. A number ofparallel paths are established 41, 43, 45 each containing a filter (forexample, as shown in FIG. 3, one low pass filter 41 and three band passfilters 43, 45, 48) that each pass a different frequency range. Theseparallel paths (that is, filters, thermistors and/or sensors) arepreferably located at the end 30 of the delivery tube 3, in a similarmanner as described in relation to FIG. 2. The parallel paths allow theheating current to be passed through a different path to the measurementcurrents. It also allows multiple measurement signals to be passedthrough the heater wire so that different properties of the gases (e.g.temperature, pressure, humidity, composition) may be measured.

The heating and measurement currents return through the heater wire 46and can be filtered through a number of measurement filters 47, 49, 57in parallel that pass frequency bands that correspond to the filters,41, 43, 45 located at the end 30 of the tube 3. The heating currenttakes a different path than the measurement currents. The measurementcurrents each take a different path depending on their frequency andthis allows each measurement current to be measured by passing itthrough a reference resistor 48, 54 or similar. Again a look up table orequation may be used to convert the voltage across the referenceresistor 48, 54 to, for example, a temperature. In the preferredembodiment of the present invention the measurement filters 47, 49, 57would be included in the humidifier 8 control circuitry.

In a further embodiment one or more of the sensing elements 55, 56 atthe end 30 of the delivery tube 3 could be replaced by a fixed impedanceto allow identification of the tube so that different control algorithmscan be used for different conduits or tubes.

FIG. 4 shows a cutaway view of a conduit 3 with a printed circuit board60 housing the parts to one of the circuits of the present inventiondescribed above with reference to FIG. 2 or 3. The circuit board 60 isconnected to the heating wires 21, 28 and as such is positioned withinthe conduit 3. In this manner, the thermistor 23 included on the board60 is exposed to the gases flowing through the conduit 3 and can providemeasurements of the properties of the gases.

The circuits and method of the present invention can be applied to anumber of applications of these technologies for humidification andbreathing circuit products. For example, the measurement of thetemperature or humidity at the end of the delivery tube (or in a patientinterface, for example, nasal cannula or mask) can be used to bettercontrol the humidifier, such that a more accurate temperature of gasescan be supplied to the patient, providing optimal patient comfort andtherapy. Additionally, other gases properties may be measured, such asthe gases pressure or gas composition near the patient.

The apparatus of the present invention eliminates the need for externalwires for sensing gases properties, as is required by the prior art.Furthermore the apparatus of the present invention only uses two pins orcontacts (as opposed to four pins as used in current heated tubeimplementations). This means the system of the present invention islikely to be more reliable as the contacts/pins are likely to be lessprone to breakage. The utilisation of the heater wire for measuringgases properties may also reduce the cost of the breathing tube 3 andassociated parts, especially if the breathing tube is to be disposable.

1-16. (canceled)
 17. An apparatus for measuring properties of gasesbeing supplied to a patient comprising: a gases supply; and at least onedelivery conduit including a heater wire for heating said conduit,wherein said heater wire is located within, around or throughout saidconduit, said heater wire extending longitudinally along said conduitfrom at or near a first end of said conduit to at or near a second endof said conduit, and said heater wire being utilized in an electricalcircuit to determine said properties of said gases, wherein theelectrical circuit comprises at least one a filter and an electricalcomponent arranged in series with the filter, wherein the filter isconfigured to pass a. particular frequency range, and wherein a voltagesignal is provided to the electrical circuit, the voltage signalcomprising at least one measuring signal, the measuring signal having afrequency corresponding to the frequency range of the filter, whereinthe measuring signal passes through the filter to the electricalcomponent, to provide for measurement of an output of the electricalcomponent.
 18. The apparatus of claim 17, wherein the measurement signalis an AC signal.
 19. The apparatus of claim 17, wherein the electricalcircuit comprises a. plurality of filters with a correspondingelectrical component arranged in series with the filter, each filterconfigured to pass a particular frequency range.
 20. The apparatus ofclaim 19, wherein the plurality of filters with the correspondingelectrical component are arranged in parallel with each other.
 21. Theapparatus of claim 17, wherein the electrical circuit is arranged inseries with the heater wire.
 22. The apparatus of claim 17, wherein themeasuring signal comprises a plurality of measuring signals, eachmeasuring signal having a frequency corresponding to the frequency rangeof an associated filter.
 23. The apparatus of claim 17, wherein thevoltage signal comprises a heating signal configured to provide power tothe heater wire.
 24. The apparatus of claim 23, wherein the electricalcircuit comprises a low pass filter, the low pass filter provides anelectrical pathway for the heating signal.
 25. The apparatus of claim24, wherein the low pass filter is provided in parallel with the atleast one filter and electrical component.
 26. The apparatus of claim23, wherein the apparatus comprises a low pass filter, the low passfilter providing a pathway to ground for the heating signal.
 27. Theapparatus of claim 23, wherein the measuring signal comprises aplurality of measuring signals, wherein the apparatus comprises at leastone adder to sum the plurality of measuring signals and the heatingsignal.
 28. The apparatus of claim 17, wherein the measuring signalcomprises a plurality of measuring signals, wherein the apparatuscomprises one or more measurement filters, each measurement filterhaving a frequency range corresponding with each filter and/or afrequency of each measuring signal.
 29. The apparatus of claim 28,wherein each measurement filter comprises an associated referenceresistor.
 30. The apparatus of claim 28, wherein each measurement filterfilters out a particular measuring signal from the plurality ofmeasuring signals.
 31. The apparatus of claim 17, wherein at least oneelectrical component comprises a sensor.
 32. The apparatus of claim 31wherein the sensor is one or more of: a temperature sensor; a pressuresensor; a humidity sensor; or a gases composition sensor.
 33. Theapparatus of claim 17, wherein at least one electrical component is anidentification element, the identification element allows foridentification of the delivery conduit.
 34. The apparatus of claim 33,wherein the identification element is a reference resistor.
 35. Adelivery conduit, the delivery conduit comprising a heater wire forheating said conduit, wherein said heater wire is located within, aroundor throughout said conduit, said heater wire extending longitudinallyalong said conduit from at or near a first end of said conduit to at ornear a second end of said conduit, and said heater wire being configuredto be utilized in an electrical circuit to determine properties of saidgases, wherein the electrical circuit comprises at least one a filter,and an electrical component arranged in series with the filter, whereinthe filter is configured to pass a particular frequency range, andwherein the electrical circuit is configured to receive a voltagesignal, the voltage signal comprising at least one measuring signal, themeasuring signal having a frequency corresponding to the frequency rangeof the filter, wherein the measuring signal is configured to passesthrough the filter to the electrical component, to provide formeasurement of an output of the electrical component.
 36. The deliveryconduit of claim 35, wherein at least one electrical component is anidentification element, the identification element allows foridentification of the delivery conduit.